It may simply be a temporary setup until a better one is ready. I've had fish abandoned on the doorstep of my hatchery and at the rear door of a lfs I used to work at.
At the lfs, the usual scenario was: "Will you buy my _____? It got too big". If we were full or the fish was too big for the shop we had to answer 'no'. Then, lo and behold, that fish would be in a bucket or box, at the back door, when we arrived to open up the next morning.
My hatchery is private property (not a walk-in store) and I've had 4 large fish abandoned here in the last year. A RBP (illegal in MS) which I fed to my AST, 2 RB Pacus, and a long-fin oscar.